This invention relates generally to The present invention relates to active balancing circuitry for capacitors and methods for their manufacture. More particularly, the invention relates to a module based active balancing circuitry for capacitors stacked in a series arrangement.
A capacitor is often designed with a maximum operating voltage rating where operation above this maximum operating voltage rating will generally lead to excessive leakage current, subsequent gas generation, and ultimately failure of the capacitor. In order to construct capacitances for higher voltage applications, capacitors of identical value and rating are stacked in a series arrangement. During charging as well as after charging of the capacitors, the total voltage divides equally across each capacitor in the stack so long as the capacitance values of each capacitor remain identical. Once charging is mostly complete and equilibrium conditions in the stack are reached, the voltage continues to divide equally across each capacitor as long as the leakage current of each capacitor is equal. In sum, unequal voltages across capacitors in such a series arrangement generally have two causes: 1) unequal capacitance values; 2) unequal equilibrium leakage current.
Several approaches have been conventionally implemented to attempt to control the voltage imbalances due to unequal capacitance values or to unequal equilibrium leakage current. In one approach, a technique known as passive balancing is used. Passive balancing involves using passive elements to equalize the voltages between the capacitors in the stack. Typically, this includes connecting equal valued resistors in parallel with the capacitors. Although passive balancing may work initially, passive balancing has a drawback of not being able to adjust for voltage imbalances due to changes in capacitance values or leakage currents over time.
In another approach, a technique known as active balancing is used. Active balancing involves using active elements to equalize the voltages between capacitors. Although this technique can be used to adjust for voltage imbalances over time, the conventional implementation of these active devices often is complicated and expensive.
Accordingly, there is a need to provide improved apparatus and corresponding methods that are easier and less expensive to implement and that can control voltage imbalances between capacitors stacked in a series arrangement, thereby, improving the capacitors life expectancy and electrical performance.